Arsenic compounds isolated and identified in cod-liver oil

A group of chemists report their identification of six arsenic-containing long …

Cod liver oil and other fish oils are often used as dietary supplements because they contain nutrients such as omega-3 fatty acids, vitamin A, and vitamin D. It has been known for some time that these oils contain arsenic compounds, but none of these molecules have been clearly identified. Without knowing the chemical structures of the arsenic compounds, speculation about their formation or toxicity would be pointless. In an Angewandte article, a group of chemists report the identification of six arsenic-containing long-chain fatty acids that were isolated from cod liver oil.

The chemists divided crude cod liver oil between hexanes and aqueous methanol to extract polar chemicals. After subjecting the polar compounds to preparative chromatography using anion-exchange and size-exclusion separation, they obtained a sample that was enriched with polar arsenolipids. Analysis of the sample showed that there were at least 15 arsenolipids present; the authors were able to ascertain the structures of six of them.

Four of the arsenolipids (A-D) contain dimethylarsinoyl groups, (CH3)2As(O)-, that replaced the terminal methyl groups in known saturated fatty acids. The other two arsenolipids (E and F) are unsaturated fatty acids that also contain the dimethylarsinoyl group. The positions of the double bonds in E and F cannot be explicitly assigned with the researchers' experimental procedure, so they placed the double bonds according to the structures of known unsaturated fatty acids that are commonly found in cod liver oil. In order to unambiguously assign the double bonds, the authors would need to isolate large enough quantities of E and F for NMR experiments.

Based on these six structures, researchers made some logical deductions about the source and toxicological relevance of arsenolipids. They suggested two possible paths to the formation of arsenolipids. First, a natural detoxification process that sequesters potentially harmful arsenic could be responsible. Second, these arsenic compounds could be produced by nonspecific biosynthesis mechanisms that normally generate essential fatty acids. Since the arsenolipids resemble essential fatty acids, the second choice seems more likely.

At present, there is no experimental evaluation of arsenolipid toxicity but, based on the proven toxicity and carcinogenicity of arsenic, some inferences can be made. Exposure to arsenic typically comes via arsenate, which is present in some drinking water. Following exposure to arsenate, humans get rid of it via urination of dimethylarsinic acid. People who ingest cod liver oil excrete dimethylarsinic acid, as well. Also, beta-oxidation of any of the six arsenolipids shown above produces dimethylarsinous acid, one of the compounds implicated in arsenic's toxicity. Clearly, arsenolipids have the potential to be toxic.

These six compounds only account for about 20 percent of the total arsenolipids in cod liver oil. Non-acidic, polar arsenic compounds account for another 20 percent of the total. The rest of the arsenolipids, 60 percent, are non-polar compounds that are soluble in hexanes. Thus, much more work is necessary before we have a complete picture of the arsenic content in cod liver oil. Even so, the current structural information allows some informed speculation regarding the origins and toxicity of these arsenolipids. Since the structures reveal that they are potentially toxic to humans, consumers should be cautious when viewing cod liver oil as a health supplement. The researchers are working on identifying more arsenic compounds in marine samples and looking further at their biological effects.

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Yun Xie
Yun Xie / Yun Xie is a contributing science writer at Ars, where she covers the latest advancements in science and technology for Ars. She currently works in scientific communications, policy, and review. Emailreenxie@gmail.com//Twitter@yun_xie